An image formation apparatus such as a copier, a printer or a facsimile is provided with a fixing device which thermally fuses a toner image transferred onto a recording sheet and which fixes the toner image.
As one configuration of the fixing device, there is known a configuration which uses a heating roller system. This heating roller system is provided to combine a fixing roller including a heat source with a pressure roller which is opposite to the fixing roller and which can rotate to be interlocked with the fixing roller while being abutted on the fixing roller to thereby act heat and pressure on the toner image.
Since the heating roller system employs rollers having a relatively high heat capacity, it takes long time to raise the temperature of the rollers up to a predetermined fixing temperature. To replace this configuration, therefore, there is proposed a configuration in which a belt of a very low heat capacity is used. According to the latter configuration, the belt is laid on a pair of rollers, the pressure roller is arranged to face one of the rollers, a heat source is incorporated in the other roller and the belt is thereby heated. This configuration is advantageous not only in that the belt can be heated in shorter time and time required to raise the belt temperature up to a predetermined temperature can be shortened but also in that a part of the extended surface of the belt is put along the peripheral surface of the pressure roller, whereby the holding and conveying region or a so-called nip width of a recording paper which carries the toner image can be increased, a heating region for the toner image is increased and fixing efficiency can be, therefore, improved.
The fixing device is provided with a constituent element that prevents the inverted transfer or so-called offset of toner in a fused state to a member on a side in contact with the toner from occurring. As such a constituent element, there is known one that applies an offset prevention liquid which serves as a mold releasing agent.
To apply the offset prevention liquid, a felt, a part of which is soaked in a liquid of an offset prevention liquid tank, is employed, and the offset prevention liquid is pumped up from the tank using the surface tension of the felt.
The method of applying the offset prevention liquid pumped up using the felt has the following disadvantage. If the felt is kept in contact with a member to which the offset prevention liquid is supplied and to which the roller or the belt corresponds, i.e., a target to which a liquid is applied (“application target”), the pump-up action of the felt is continued and the offset prevention liquid becomes excessive on the surface of the application target, with the result that a part of the recording sheet which carries the toner image may possibly be damaged.
To overcome this disadvantage, a configuration in which the felt is brought into contact with the application target only when the offset prevention liquid on the application target is scant is conventionally proposed.
If the felt is provided in such a manner as to come in contact with and be separated from the target to which the offset prevention liquid is supplied, the amount of the off set prevention liquid impregnated into the felt sometimes changes by the evaporation of the offset prevention liquid depending on the amount of impregnated offset prevention liquid or a standby time required until the felt contacts with the application target. If the amount of the offset prevention liquid impregnated into the felt is large, in particular, the offset prevention liquid is excessively supplied to the felt compared with an instance in which the amount of the impregnated offset prevention liquid is small. As a result, when the felt contacts with the application target, the offset prevention liquid has been excessively supplied to the application target. If so, every time the felt contacts with the application target, then the offset prevention liquid is excessively applied to the member, the application target has sections to which the excessive offset prevention liquid is applied and those to which the offset prevention liquid is applied less excessively and the nonuniform application of the offset prevention liquid occur intermittently to the application target. This phenomenon causes the same result as that if the offset prevention liquid is transferred onto the sheet on which the toner image to be fixed is carried and spots are generated on the sheet. Consequently, a corrugation phenomenon occurs to the sheet, disadvantageously making the appearance of the sheet unfavorable.